In recent years, various techniques of spread spectrum radar apparatuses using spread spectrum methods have been suggested (for example, see Patent Reference 1).
The spread spectrum radar apparatuses spread-modulate narrow band signals to wide band signals using transmission pseudo-noise codes. The spread spectrum radar apparatuses transmit the spread-modulated wide band signals as radar waves. Then, the spread spectrum radar apparatuses receive, as reception signals, reflected waves obtained by reflecting the transmitted radar waves from objects. Then, the spread spectrum radar apparatuses spread demodulate the reception signals to correlation signals using reception pseudo-noise codes. Based on the correlation signals obtained through the spread demodulation, the spread spectrum radar apparatuses calculate the presence or absence of objects, distances to the objects, and relative velocities of the objects.
Here, the transmission pseudo-noise code is a pseudo-noise code, such as an M-sequence code and a Gold sequence code. Here, the M-sequence code is used as an example due to its superior autocorrelation characteristic. Furthermore, the reception pseudo-noise code is a pseudo-noise code obtained by delaying the transmission pseudo-noise code. In other words, the reception pseudo-noise code is a pseudo-noise code obtained by shifting a phase of the transmission pseudo-noise code by a chip count corresponding to the delay time. The delay time corresponds to a difference between a transmission time period of the radar waves and the reception time period of the reflected waves.
Next, the detection principle behind the spread spectrum radar apparatuses will be described with reference to drawings.
FIG. 1 illustrates an outline of the detection principle behind a conventional spread spectrum radar apparatus. Here, a scan range is from 1 to 100 meters inclusive, and the resolution is 1 meter as an example.
In order to cover the scan range under the assumption, the spread spectrum radar apparatus generates a reception pseudo-noise code to correspond to a transmission pseudo-noise code 11 by displacing 1 to 100 chips of the reception pseudo-noise code in ascending order. Here, since a width of one chip in a code determines a resolution, a phase shift by 100 chips becomes necessary, and a cycle of pseudo-noise codes needs not smaller than 100 bits.
When codes are displaced by 100 chips as in a reception pseudo-noise code 16, the process returns to the initial state, and the spread spectrum radar apparatus repeatedly generates reception pseudo-noise codes by displacing codes of 1 to 100 chips in ascending order again. Here, a time period from displacement of 1 to 100 chips corresponding to the scan range to restoration to the initial state is defined as one radar scan cycle.
More specifically, the spread spectrum radar apparatus generates a reception pseudo-noise code by displacing the transmission pseudo-noise code 11 by one chip or a unit not larger than one chip. Then, the spread spectrum radar apparatus performs a correlation between the generated reception pseudo-noise code and a reception signal 13. Then, when a phase of the generated reception pseudo-noise code matches a phase of the reception signal 13, a correlation signal peaks. On the other hand, when the phases do not match each other, a correlation signal does not peak. A synchronous state in which the phases match each other and a non-synchronous state in which the phases do not match each other will be hereinafter described.
For example, when the spread spectrum radar apparatus performs a correlation between a reception pseudo-noise code 14 and the reception signal 13, the correlation signal does not peak as being in the non-synchronous state. On the other hand, since a reception pseudo-noise code 15 and the reception signal 13 are in the synchronous state, the correlation signal peaks. Here, the reception pseudo-noise code 14 is a reception pseudo-noise code obtained by shifting a phase of the transmission pseudo-noise code 11 by one chip. The reception pseudo-noise code 15 is a reception pseudo-noise code obtained by shifting the phase of the transmission pseudo-noise code 11 by a predetermined count of chips.
FIG. 2 illustrates an outline of a radar signal that has been spread-demodulated by a conventional spread spectrum radar apparatus.
As illustrated in (a) in FIG. 2, when phases match each other (i.e. a synchronous state), a radar signal peaks. When phases do not match each other (i.e. non-synchronous state), a radar signal does not peak. Furthermore, when a single object is present, only a peak appears per cycle of codes. When plural objects are present, plural peaks appear. Thereby, the spread spectrum radar apparatus can detect each object by detecting a peak from the received reflected waves. Here, the peak intensity appearing in the radar signal is determined by a degree and a distance in and at which a radar signal is reflected from a target object.
As such, the spread spectrum radar apparatus specifies a count of chips corresponding to a phase shift between a transmission signal 12 and a reception signal 13 using the transmission pseudo-noise code 11 and the reception pseudo-noise code 15 so that a delay time corresponding to the specified count of chips corresponding to the phase shift can be specified. Furthermore, a distance to an object can be calculated through calculation of a distance corresponding to the specified delay time. Here, the transmission signal 12 is radar waves transmitted from the spread spectrum radar apparatus. The reception signal 13 is reflected waves obtained by reflecting the radar waves from the object.
However, there are cases where a peak may appear which may cause false detection of detecting other than the reflected waves obtained by reflecting the radar waves from a target object, depending on a pseudo-noise code and others to be used.
In order to solve the problem, the spread spectrum radar apparatus including a processing unit that reduces false detection of a radar signal as a target detecting unit 9 as illustrated in FIG. 3 has been disclosed (for example, see Patent Reference 2). The spread spectrum radar apparatus can reduce the false detection by setting a threshold for detecting a target object using an average value and a standard deviation of the radar signals.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 5-93776    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2005-207932